Sea Urchins and Sand Dollars, Class Echinoidea: Friends and associates who know me to be an avid diver frequently ask whether I'm concerned with potential encounters with sharks, rays, giant squids and the like. My standard reply is that hour per hour spent the most dangerous activity we all engage in is driving on the freeway. In all honesty, in the way of moments spent underwater, other than your dive buddy, Sea Urchins are the most realistically harm/hurtful organisms.

Urchins are common fare in the marine aquarium trade. They are useful and interesting scavengers and algae eaters, but just as with underwater meetings, they present considerable risk.

Classification: Taxonomy, Relation With Other Groups

Urchins and their allies the sand dollars and heart urchins make up the Class Echinoidea in the phylum Echinodermata. You're familiar with this phylum's other four living classes, the Sea Stars, Brittle stars, Sea Cucumbers and Sea Lilies & Feather Stars. Collectively the echinoderms or spiny-skinned animals are grouped as radially symmetrical, with a water-vascular system (ambulacral) responsible for their peculiar locomotory tube feet. They have a true body cavity (coelom) supporting a calcareous internal skeleton...

The class Echinoidea including the Urchins are discoidal, ovate or globose echinoderms having bodies covered with spines and no arms. The name echinoidea is actually Greek for "like a hedgehog" referring to these spines. Their mouths are aboral, that is, directed against the substrate. Distributed between the spines are pedicellariae, specialized tube feet used for cleaning and defense. Some of these are termed globeriferous pedicellariae (now that's a mouthful!) and contain poison glands. About 800 species have been described.

They come in brown, black, purple, green, white, red and multicolored. Most are 6-12 centimeters in diameter; some Indo-Pacific species attain 36 cm. in diameter! Some have very sort spines, some in the genus Diadema on tropical reefs have spines that may be more than 36 cm. in length (Ouch!).

Irregularly shaped echinoids appear bilateral. These include the Heart and Cake Urchins and Sand Dollars. All are adapted for burrowing in sand, possess much smaller and more numerous spines. Irregular urchins all feed on minute organic particles in the sand in which they burrow. Our local common Californian Sand Dollar Dendraster exocentricus feeds almost exclusively on suspended particles, particularly diatoms. Non-"regular" urchins are rarely offered in the trade.

Species Sometimes Seen/Used In the Aquarium Interest & Not:

Family Diadematidae: Astropyga, Diadema, Echinothrix.

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